The invention relates to focusing devices applying a magnetic field to the electron beam of a scanning beam apparatus such as a cathode ray tube, for focusing the beam at a point on a screen or other surface oriented normal to the beam.
Electrons emitted from the cathode of a CRT are accelerated toward the screen, but tend to diverge as they travel due to electrostatic repulsion between the electrons. The electrons can be forced into a tight beam using magnetic fields. Typically, means producing a steady state or static magnetic field and means producing a varying or dynamic magnetic field are superimposed for focusing. The dynamic portion of the field varies as a function of the scanning position of the beam on the screen or other surface on which the beam is incident.
The focusing coils of a scanning electron beam apparatus form a magnetic lens having a focal length. However, the screen or other display device is substantially planar rather than spherical, and as a result the distance between the focusing device and the point at which the beam is incident on the planar display device varies as a function of vertical and horizontal position of the scanning beam. The distance is greater at the edges of the screen and less in the center. In order to accommodate the difference in beam path length (i.e., to converge the beam at a point as the beam scans through areas of varying path length), the dynamic focusing coils are driven with a current signal which is shaped as a parabola at the scanning rate of the main deflection circuits.
The dynamic focusing current signal adds to or subtracts from the magnetic field applied by the static field generator, which may comprise permanent magnets or a coil carrying a steady state direct current. In a device wherein the static and dynamic signals are applied to the same coil, the steady state and time varying currents can be summed.
The dynamic portion of the focusing deflection is such that the level of the focusing deflection field due to the sum of the static (e.g., permanent magnet) field and the dynamic parabola is at a maximum when the beam is directed at the center of the screen (corresponding to a shorter focal length) and at a minimum when the beam is directed at the edges of the screen (corresponding to a longer focal length). The beam converges at a point on the screen or other display apparatus regardless of the X-Y position at which the beam is incident while scanning.
The dynamic part of the focusing device is generally controlled by a current-controlled amplifier, i.e., an amplifier which produces a parabolic current through a focusing coil coupled to the output of the amplifier in response to an input at the scanning frequency. The input is typically coupled to a parabola voltage generated synchronously with the flyback pulse which resets the main deflection circuits after a horizontal scan. Current-controlled amplifiers, however, dissipate a great deal of power.